Could this be a Lanceolated Warbler, which is very uncommon in our region??? Because living in Southern Germania I am not common with this species. I recovered this recording between older recordings of mine and again I wondered if that bird really could be only a Common Grasshopper Warbler, because it sings so much higher frequented than Locustella naevia, which I often heard. And comparing it with a recording of H.-H. Bergmann and recordings in xeno-canto (even looking at the sonagrams) of Locustella lanceolata I noticed, that they are as high frequented as my recording..
The bird sang in high wild herbs and bushes at the edge of a little field wood, which belongs to a little wetland (on the other side fields). The singer seemed to be very near, but I couldn't see him. I only heard him that special day.

The song heard from a nearby Indian Tamarind tree(Tamarindus indica) and could be heard for almost 30 mins. Bird was not visible due to dense foliage. The area is residential area with scattered vegetation. The song is much clear after 0:26.